MP4 to WEBM

[Undertow]

I've done this countless times but now I'm having trouble. The .mp4 is 16 seconds long but when I try and convert it to a .webm it makes it 42 hours long. I restarted my computer and instead of 42 hours long it's 32 minutes long. What's weird is that after the 16 seconds of actual content there isn't a blank screen for 42 hours/32 minutes, it just stops playing after 16 seconds like usual and if I try and skip past 16 seconds it just stops the video as though it finished playing. Any ideas?